I would say that this is not necessarily an issue, depending on how one looks at their game. For me personally... gaining an ability that is the same as another class's is not a problem if there isn't a person at the table playing that class (and thus not have to worry about stepping on toes). So for instance, if no one is playing a Monk in a particular campaign, I wouldn't have an issue if a player wanted to create a Feat for their 4th level selection that duplicated the functionality of a Monk class feature of an equal or lower level-- like if someone wanted the have Deflect Missiles for instance because it really added to their character and its concept. To my mind, this would be a one-off decision that just works for this one isolated character in this one isolated game, and thus I'm good with it. But I'm not going to add this special dispensation to my sheet of house-rules for the future so that others can take it themselves in subsequent campaigns.
But this is why I say it depends on how folks look at their game, because I'm fairly certain that there are DMs out there who want a consistency to their game and world and would believe that any rule like this they allowed in one game must now be a rule they allow in all subsequent games. I'm sure for them the rules define the reality and to just arbitrarily make up rules to then throw away when you're done is not what they look for in a system. Which I totally respect. But it does make it such that we can't call it a black or white issue. Using other class's features might work for some, but not for others. And each DM will need to determine it for themselves.
